Why do fish like the shelf edge?

March 31, 2010

During recent research cruises, a team led by Dr Jonathan Sharples, who is based at the Liverpool site of the newly formed National Oceanography Centre, discovered why the mackerel (and other fish species) like the edge of the continental shelf.

About 200 miles southwest of the Isles of Scilly the depth of the sea suddenly increases from 200 metres down to the abyssal 3 – 4 kilometres of the NE Atlantic Ocean. This is the edge of the continental shelf. It is a site of some of the most intense fishing activity throughout the Celtic and Irish Seas, with fishing vessels hunting for large stocks of spawning mackerel.

In summer the surface 30 metres of the ocean is warmed up by the sun, and becomes separated from the deeper, colder water by a narrow zone with a sharp temperature transition called the thermocline. Waves can travel along this thermocline, similar to waves travelling along the sea surface but much longer and larger. These ‘internal’ waves are set up by the flow of the tides off the shelf. As the waves cross into the shallower water on the shelf they break, like swell waves breaking as they approach a beach. As they break, they generate lots of turbulence which mixes nutrients from the deep water up towards the surface. The phytoplankton, single-celled plants that form the base of the marine food chain, thrive on this supply of nutrients leading to a band of chlorophyll aligned along the shelf edge.

Figure 1 is of the sea surface chlorophyll around the UK in June. There’s a band of high chlorophyll along the 200 metre isobath (shelf edge). The chlorophyll tells us where the phytoplankton are (they use the same green chlorophyll pigment to photosynthesise as terrestrial plants).

Importantly, the phytoplankton that grow at the shelf edge are much larger than those growing in the adjacent Celtic Sea and in the open NE Atlantic Ocean. And that is why the fish go there. The large phytoplankton cells provide ideal food for the fish larvae just as they hatch from their eggs. The distributions of the fishing vessels, the fish, the fish eggs and larvae, and the large-celled phytoplankton are all aligned with the place at which the internal waves break along the shelf edge. The physics of the tides creating the internal waves is a fundamental control on the survival of this commercially important fishery.

Figure 2 shows a close-up (box in the left panel) of sea surface temperature and sea surface chlorophyll. The shelf edge is cool because of the cold water with nutrients being mixed towards the sea surface. The chlorophyll is aligned with that band of cool water.

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